In upstream reaches of the Tama River, musty odor substance (2-methylisoborneol (2-MIB)) began to be detected in 2008 and the concentration has been increasing thereafter. Then, in 2012, 2-MIB in raw water of a water treatment plant reached 210 ng/L. It was suspected that cause of musty odor was benthic cyanobacteria (Phormidium) which attached to stones on the riverbed. However, identification of the benthic Phormidium by microscopic observation had been difficult, thus genetic analysis was carried out. In genetic analysis, almost full-length 16S ribosomal DNA and internal transcribed spacer region gene of benthic Phormidium strains, which include the strain isolated from the Tama River, a strain in the Yoshino River (350 km away from the Tama River) and standard strains, were sequenced and compared. From homology search of these sequences, 2-MIB producing Phormidium in the Tama River was classified into Phormidium autumnale. Furthermore, it was found that P. autumnale in the Tama River was the same species in the Yoshino River. In addition, the result of in vitro cultivation shows that the P. autumnale inhabiting the Tama River thrives and produces more 2-MIB at high temperature, but it can grow and produce 2-MIB even at low temperature.

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